1. Field of the Invention
The present invention relates to an optical transmission apparatus and a method of determining an optimal optical transmission route advantageously applicable to the routing system of, among various communication apparatuses, an optical transmission apparatus.
2. Description of the Background Art
Networks in general include a ladder type network and a ring type network. A ring type network includes ADMs (Add/Drop Multiplexers) for multiplexing optical signals to be output to ring transmission paths and demultiplexing optical signals input from the transmission paths. ADMs using optical fiber cables as the ring transmission paths are referred to as OADMs (Optical ADMs). An OADM differs from an ADM in that it has a bridging or through function between two transmission paths in addition to a multiplexing and demultiplexing function. Further, an OADM has a function of selecting a route along which a signal should be transmitted.
It is a common practice with an OADM to execute route selection mentioned above by default setting based on an APS (Automatic Protection Switching) system or by a software strap effected before installation or from a remote site. This kind of route selection has actually been applied to a SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) ring that implements an economical digital network.
A routing protocol prescribed by the IETF (Internet Engineering Task Force) standard is another implementation available for route selection. It has been proposed to apply this routing protocol to the selection of the transmission route of an optical transmission apparatus in such a manner as to automatically find out an optimal transmission path. The above routing protocol is RIP/OSPF (Routing Information Protocol/Open Shortest Path First).
To set up a route from one client's apparatus to another client's apparatus with the routing protocol, each optical transmission apparatus reports its link state, or connect information, to the entire network. The client's apparatuses transfer information of the link states among them to thereby establish a topological database. Each transmission apparatus determines an optimal route by using the routing protocol by referencing the topological database. In practice, a metric, which is a parameter to be set by a user, is used for determining an optimal route. Typical of metrics is the number of hops between optical transmission apparatuses that are respectively connected to a source and a destination. The number of hops is representative of the number of optical transmission apparatuses which IP (Internet Protocol) packets are expected to pass. Usually, one rout including a smaller number of hops than the other routes is determined to be an optimal route.
It sometimes occurs that a plurality of routes including the same number of optical transmission apparatuses, which correspond to nodes, are selected as optimal routes. All of such routes satisfy the above-described optimal route condition and are therefore expected to be shortest. In practice, however, route selection depends on conditions other than the number of hops as well. For example, the length of a transmission path between central offices differs from one route to another route. Further, some nodes execute repeating while other nodes do not. Moreover, an error correcting function referred to as FEC (Forward Error Correction) is used to improve the bit error rate of information, as the case may be. When such route conditions other than the number of hops are taken into account, a transmission distance and an actual transmission ability are not always consistent with each other. Consequently, the routes including the same number of hops sometimes noticeably differ in actual transmission conditions from each other.